1. Field of the Invention
The present invention relates to a process for the preparation of a high .alpha.-type silicon nitride powder suitable for the production of a sintered silicon nitride body valuable as a high-temperature structural material.
2. Description of the Related Art
According to the thermal decomposition of silicon imide, .alpha.-type silicon nitride having a high purity can easily be obtained. However, the grain of the product is mainly of a needle or columnar shape, and when the product is used as a sintering material, the resulting sintered, molded body has a very low density and a sintered body having a high density cannot be obtained.
As means for overcoming this defect, we previously proposed a process for the preparation of a silicon nitride powder by the thermal decomposition of a nitrogen-containing silane compound, wherein the temperature-elevating rate is controlled to at least 15.degree. C./min in the temperature range of from 1,350.degree. to 1,550.degree. C. (Japanese Unexamined Patent Publication No. 58-55,315) and another process for the preparation of a silicon nitride powder in which a nitrogen containing silane compound having a powder bulk specific gravity of at least 0.1 g/cm.sup.3 as silicon is heated (Japanese Unexamined Patent Publication No. 58-55,316). Powders obtained according to these processes have a high purity and a very fine size, and show an excellent sintering property and excellent high-temperature characteristics. However, since the value of the fracture toughness is low, the strength of these powders at normal temperature is almost the same as that of the conventional silicon nitride powder.